Ink jet printers, and in particular, drop-on-demand ink jet printers having print heads with acoustic drivers for ink drop formation are well known in the art. The principle behind an impulse ink jet of this type is the generation of a pressure wave in an ink chamber and subsequent emission of ink droplets from the ink chamber to a nozzle orifice as a result of the pressure wave. A wide variety of acoustic drivers have been employed in ink jet print heads of this type. For example, the drivers may consist of a transducer formed by piezoceramic material bonded to a thin diaphragm. In response to an applied voltage, the diaphragm displaces ink in the ink chamber and causes a pressure wave and the flow of ink to one or more nozzles. Other types of acoustic drivers for generating pressure waves in ink include heater-bubble source drivers (so called bubble jets) and electromagnet solenoid drivers.
Known ink jet printers typically combine an ink jet print head from which drops of ink are ejected toward print media with a reservoir for supplying ink, including plural colors of ink, to various nozzles of the ink jet print head. Also, it is common to shuttle or scan ink jet print heads transversely across print media as the print media is being printed by the ink jet print head.
In a typical application, print media is delivered to a drum and secured to the drum for subsequent printing by a mechanism such as a clamp which grips one edge of the print media. As the drum rotates past the print location, ink is ejected from the ink jet print head onto the print media to accomplish the printing. In a typical printing arrangement, the print media is in effect printed in rows of pixels, each pixel or picture element having a predetermined size and height. The height of the pixel element corresponds to a distance along the print media in the direction of rotation of the drum.
For accurate printing, precise control of the transportation of print media through a printer is required. Any slack in the print media during printing and errors in advancing the print media through the print media transport path can cause a degradation in print quality.
One prior art printer is described in U.S. Pat. No. 4,386,771 to Lakdawala. This patent describes a gripping bar which opens to receive the edge of a sheet of media and closes to hold the sheet onto a drum. The drum is initially rotated in a first direction. The use of a printing means such as a moving head electrographic printer or an array of stationary thermoprinters is mentioned. To eject a sheet of print media from the drum, the direction of drum rotation is reversed with the free edge of the surface of the sheet lifting off the drum by the beam strength of the sheet, the free edge being the edge opposite the gripped edge. The free edge is delivered to a sheet stripper which guides the sheet away from the drum, the stripper being shown in this patent as being located close to the 12 o'clock position of the drum.
U.S. Pat. No. 4,317,138 to Bryan et al. discloses a method and apparatus for facsimile sheet handling in which a first edge of a sheet is transported to a clamp on a drum. The drum is rotated in a first direction during scanning. A roller is positioned against the drum and the drum rotation is reversed to remove the sheet from the drum. This roller is spaced from the drum during the scanning operation.
In each of these references, inasmuch as the clamp is the only mechanism understood to be used to hold the sheet on the drum during scanning, inaccuracies in printing can result.
Another exemplary printer is shown in U.S. Pat. No. 4,581,618 to Watanabe, et al. In Watanabe, print media is fed by a roller mechanism to a first pinch roller which bears against a drum. This drum is described as a platen of high friction material, such as rubber. Soft rubber rollers would not have a radius within a narrow precision, particularly at locations where engaged by other rollers. A motor 10, described as a "feed pulse" motor, is used to drive the drum and the paper transport mechanism of the printer. An elaborate multiple gear drive system is used to couple the motor to the drum and other paper transport rollers of the system. At the appropriate time, print media is transferred from the first roller to the drum, with the print media being retained on the drum by a movable pinch roller illustrated in this patent as having a gap in its center. The movable pinch roller retains the paper on the drum by torque established by a friction clutch. During printing, the drive mechanism is described as rotating the drum through a predetermined angle after the printing of each line of print.
Because gears are used in the paper transport mechanism of this device, backlash may occur which can result in inaccuracies of print media positioning during the printing operation. Also, since gears typically have very few teeth which engage one another at any given time, variations in the configuration of particular teeth on a gear translate to inaccuracies in the transportation of print media through the printer. In addition, a drive mechanism relying on a multiplicity of gears is subject to wear and mechanical failure.
U.S. Pat. No. 4,707,704 to Allen, et al. mentions the use of a stepper motor for driving a drum of an ink jet printer. In this patent, the stepper motor is shown coupled directly to the shaft of the drum. With this construction, the inertia of the drum is applied directly to the motor shaft and can cause a very poor inertia match between the motor and the load on the motor. In addition, this patent discloses a print media transport path in which a roll of print media is positioned within a drum and moved by an internal roller, including one covered with resilient material, to the exterior of the drum for printing purposes.
Prior art printers have also employed numerous types of clamps for securing print media to a drum during printing operations. U.S. Pat. No. 4,815,870 to Sparer, et al. discloses a sheet clamp for a thermal printer drum in which the clamp is positioned within a recess of the drum. The drum is rotated in one direction during printing and in a reverse direction during sheet ejection.
The use of pressure fixing rollers for fusing or spreading phase change or hot-melt ink on print media is also known. Japanese Patent No. 18,351 to Moriguchi, et al., U.S. Pat. No. 4,745,420 to Gerstenmeier and U.S. Pat. No. 4,889,761 to Titterington, et al. are examples. Other examples of prior art image fixing apparatus including rollers are described in U.S. Pat. Nos. 3,293,059 to Sole, 3,566,076 to Fantuzzo and 4,568,949 to Muranaka. These existing fusing rollers typically apply an extremely high loading force along the line of contact between fusing rollers, such as 100 lbs. per lineal inch. In this case, for a ten inch long roller, 1,000 lbs. of force must be applied to the rollers.
Although a number of ink jet printers are known, a need exists for an improved ink jet printer which is capable of overcoming these and other disadvantages of the prior art.